linux-kernel - RE: [PATCH] ad7877: fix spi word size to 16 bit

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Message-ID: <0D753D10438DA54287A00B0270842697636F0E1458@AUSP01VMBX24.collaborationhost.net>
Date:	Mon, 17 May 2010 19:18:28 -0500
From:	H Hartley Sweeten <hartleys@...ionengravers.com>
To:	Mike Frysinger <vapier.adi@...il.com>,
	Oskar Schirmer <os@...ix.com>
CC:	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Daniel Glöckner <dg@...ix.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-input@...r.kernel.org" <linux-input@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oliver Schneidewind <osw@...ix.com>,
	Michael Hennerich <michael.hennerich@...log.com>
Subject: RE: [PATCH] ad7877: fix spi word size to 16 bit

On Monday, May 17, 2010 4:50 PM, Mike Frysinger wrote:
> 2010/5/17 Oskar Schirmer:
>> On Sun, May 16, 2010 at 15:25:34 -0400, Mike Frysinger wrote:
>>> On Sat, May 15, 2010 at 14:15, Oskar Schirmer wrote:
>>>> On Thu, May 13, 2010 at 00:53:35 -0700, Dmitry Torokhov wrote:
>>>>> On Fri, May 07, 2010 at 02:23:07PM -0400, Mike Frysinger wrote:
>>>>>> On Fri, May 7, 2010 at 05:41, Daniel Glöckner wrote:
>>>>>>> On 05/06/2010 08:26 PM, Mike Frysinger wrote:
>>>>>>>> i think it'd be a better idea to do something like:
>>>>>>>>   if (spi->bits_per_word != 16) {
>>>>>>>>     if (spi->bits_per_word) {
>>>>>>>>       dev_err(&spi->dev, "Invalid SPI settings; bits_per_word must be 16\n");
>>>>>>>>       return -EINVAL;
>>>>>>>>     }
>>>>>>>>     spi->bits_per_word = 16;
>>>>>>>>     spi_setup(spi);
>>>>>>>>   }
>>>>>>>
>>>>>>> There is no way to set bits_per_word using struct spi_board_info. The
>>>>>>> description of that structure in spi.h explicitly lists the wordsize as
>>>>>>> one of the parameters drivers should set themself in probe().
>>>>>> >
>>>>>>> Only struct bfin5xx_spi_chip allows to set this value in the board code.
>>>>>>
>>>>>> an obvious shortcoming in the SPI framework that should be fixed, but
>>>>>> that doesnt make any difference to the above code now does it ?  it'll
>>>>>> operate correctly regardless of the SPI bus master.
>>>>>
>>>>> So is the updated patch coming?
>>>>
>>>> The basic question I see is, whether it is in the
>>>> responsibility of ad7877 to check a wrong setting
>>>> possibly caused in board specific code. If so,
>>>> then the proposal by Mike should be used, but if not
>>>> so, it would introduce unneeded code.
>>>>
>>>> Remember: both versions end up in correctly setting
>>>> bits_per_word, with the difference merely in feedback
>>>> level.
>>>
>>> imo, unsupported board settings should always be detected & rejected.
>>> all SPI master drivers do this (detect & reject unsupported SPI slave
>>> settings).
>>
>> please note, that bits_per_word is not a board setting,
>> it's a demand of the device. consequently, there is no one
>> to set unsupported values and thus none to be detected.
>>
>> the only architecture setting bits_per_word thru spi_chip
>> is blackfin, but I cannot see a good reason, why the board
>> settings should engage with a fixed demand of the device?
>
> you're telling me that every single SPI device out there can only ever
> operate in one specific bit length or lacks optional settings ?  i
> find that hard to believe which means it does make sense to let the
> boards pick a length when appropriate.
>
> the board settings also function as default setup values so when using
> generic things like the spidev driver, there is no kernel driver to
> request the desired bitmode.
>
> the simple code i posted addresses your concerns as well as reject
> invalid settings (wherever they may originate).  if you're not going
> to post an updated patch, i'll take the simple one Michael committed
> and post that.

Just my 0.02, I haven't been following this thread very closely...

I believe it's the spi "device", i.e. the protocol, drivers responsibility
to supply the bits_per_word that it will operate with.  The spi master
driver simply validates it if can support the requested size.  If the
bits_per_word is left at the default (0) it indicates that the protocol
words are eight bit bytes.

If a protocol driver can support multiple word sizes it should probably
also support a "setup" callback during the driver probe so that the
underlying board support can provide the desired word size.  This is what
the libertas_spi driver does (drivers/net/wireless/libertas/if_spi.c).
That driver does nothing with the bits_per_word value but the callbacks in
arch/arm/mach-pxa/cm-x270.c and em-x270.c do.  They both set the bits_per_word
to 16 and then call spi_setup.  They also probably should be checking the
return value of spi_setup to make sure that the requested mode is supported
by the master driver, but they probably don't since it is already known...

If the ad7877 "only" supports 16-bit word sizes it should be setting the
bits_per_word to indicate this.  If the underlying master driver does not
support that word size it will return an error when spi_setup is called.

Just my 0.02... 

Regards,
Hartley